1. Field of the Invention
Embodiments of the invention relate generally to the field of containing high-temperature alkali-containing environments. More particularly, an embodiment of the invention relates to containing a high-temperature alkali containing environment using a MgAl2O4 spinel refractory containment liner. Other embodiments of the invention relates to a MgAl2O4 spinel refractory brick and a method of making a MgAl2O4 spinel refractory brick.
2. Discussion of the Related Art
Gasification of black liquor or biomass material containing alkali compounds is an attractive means of recovering inorganic material while generating heat and pyrolizing the organic material which typically is used as syngas for further processing or generating electricity. Finding containment materials for successful, long term operation are needed to enable commercial development of this process. Degradation of containment materials in pilot-scale and demonstration-scale gasifiers has been a serious obstacle to commercialization of black liquor gasifiers.
When compared to conventional black liquor recovery boilers, combined cycle gasification offers pulp mills the potential to increase energy recovery and reduce gaseous emission while still efficiently recycling the pulping chemicals. Unfortunately, industrial implementation of black liquor gasification has been limited by the lack of refractory and metallic containment materials that survive the prolonged exposure to the gasification environment and molten salts.
If black liquor gasification were fully implemented, then 278 million dry tons of wood processed annually by the U.S. pulp and wood product facilities could potentially be gasified to generate up to 8 gigawatts of electricity from sustainable, renewable raw materials by the year 20201.
Many companies have worked on black liquor recycling gasification processes but none have been sufficiently successful for widespread commercial implementation. Industry efforts have centered on two primary processes. A low-temperature (below the melting point of alkali metal salts) gasification process utilizes a fluidized bed. Unfortunately, there are issues with carbon conversion, bed agglomeration and formation of tars2. The other process is a high-temperature (above the melting point of alkali metal salts) process. In this case, there are problems associated with containment of the molten salts3,4. While an oxygen-blown high-temperature high-pressure (HTHP) gasification process is the ultimate goal, the initial development is at near atmospheric pressures or high-temperature low-pressure (HTLP).
Chemrec AB, a Swedish company, is leading the development of the high temperature process3,4. Ultimately, the process is envisioned to operate at about 30 bars pressure (HTHP) with sub stoichiometric oxygen used to gasify the organic components of the black liquor. A near-atmospheric pressure version is also being developed as a supplement for or as a booster for recovery boilers. The operating temperature would be around 950° C. with hydrogen and carbon monoxide as the principal gaseous reaction products.
The Frövi Booster Demonstration Plant was build at AssiDomän's Frövifors mill in 1991 and taken into full operation later that same year. The system consisted of a 7 tds/24 h air-blown gasification reactor, a quench system and an associated gas cooler. The Frövi Booster was taken out of operation when in 1996 the mill no longer needed the extra capacity. The facility gasified black liquor on a commercial scale and produced green liquor of acceptable quality. The Frövi Booster experienced and solved a number of technical problems over the years. When the gasifier was finally shut down, the one issue that needed further development was the identification of a suitable refractory material to line the gasifier. Several alternatives were tested, and the one selected for further use in the next generation of plants had a life expectancy of at least one year.
Between 1994 and spring 2000, Chemrec operated a pressurized black liquor gasification pilot plant. The plant was located at the STORA Skoghall paper mill near Karlstad, Sweden, and consisted of a refractory-lined entrained flow reactor, quench system and counter-current gas cooler/condenser.
The New Bern commercial gasifier is a 300 tds/24 h unit located at Weyerhaeuser's New Bern mill in North Carolina, USA. The basic design of this facility was essentially a scale-up of the Frövi unit and started up in December 1996. The New Bern gasifier system is air-blown and operates at roughly 950° C. and slightly above atmospheric pressure. The quench system has condensate sprays to cool the gas and is followed by a 3-stage scrubber. The scrubber first cools the gas, during which most of the vapor condenses out. Hydrogen sulfide in the gas is absorbed by weak wash in the second stage. The final stage removes any residual alkali particles before the gas is fired in a power boiler.
Refractories from the Swedish HTHP black liquor gasifier operating during the 1990s and from Weyerhaeuser's New Bern gasifier during its first few years of operation have been characterized5,6. These studies showed that reaction with molten sodium salts of the bonded alumina-silica refractories used as the initial lining in the New Bern gasifier produced sodium-containing corrosion products.
Analyses performed on the exposed fusion-cast α-/β-alumina refractory samples removed from both the Swedish HTHP gasifier and the second lining used in the New Bern HTLP gasifier revealed the formation of NaAlO2 resulting from reaction between Na2CO3 and Al2O37. In the following reactionsAl2O3(s)+Na2CO3(l)→2NaAlO2(s)+CO2(g) and  (1)4NaAlO2(s)+5 H2O(l)→4NaAlO2.5/4H2O(s)  (2)the volume per mole Al changes from 21.2 for Al2O3 to 49.4 to 74.0 cm3/mole for NaAlO2 and NaAlO2.5/4H2O respectively. Thus a significant volume change for Al2O3 is associated with the formation of NaAlO2 or NaAlO2.5/4H2O. When bricks removed from the gasifier remain outside for a few months, a layer of Trona (Na3(HCO3)2(CO3).2H2O) forms on the surface along with minor amounts of Na2CO3(H2O) and Na2Al2O4.6H2O6. This confirms the presence of soluble sodium aluminates within the removed refractory.
In the New Bern gasifier system, black liquor, steam and air are injected into the top of the cylindrically shaped, refractory-lined vessel of the gasifier and pyrolyzed. The liquor and steam are injected through a specially designed spray nozzle while, simultaneously, preheated air is injected through a windbox with angled vanes that impart a swirl pattern to the air flow. The amount of air/steam injected is substoichiometric such that only sufficient reaction occurs to maintain the operating temperature and provide the energy necessary for the reduction of sodium sulfate. When the black liquor is injected into the reactor vessel, the water is volatilized, the organic components are degraded by oxidation or pyrolysis, and the sodium sulfate is reduced, primarily to sodium sulfide and hydrogen sulfide.
The bottom of the cylindrical vessel consists of a refractory cone arrangement that significantly reduces the effective vessel cross-section and directs the flow of the product gas and molten salts downward through the quench sprayers. The inorganic salts drop into the green liquor tank where they dissolve in the aqueous solution in the bottom of the vessel while the product gas is removed from the vessel through a side port. In a BLGCC, this product gas would be cleaned and routed to a gas turbine where it would serve as fuel. The environment inside the gasifier vessel is around 950-1000° C. and consists of gases including several corrosive species including molten alkali salts. This environment is so hostile that no commercially available alloy has been found that can survive for any extended period. Consequently, a refractory lining has been employed for the top dome, the cylindrical barrel and the lower cone, but it has been found that most refractories have a limited lifetime in this environment.
The original New Bern mullite-based refractory liner experienced significant materials degradation after 6 months of operation and was replaced with fusion-cast alumina. This alumina lining experienced materials degradation after a year of operation and significant expansion that led to a shutdown in January 2000.
The current design utilizes a two component lining with a more corrosion resistant refractory as the inner lining and an outer lining that has less corrosion resistance but better thermal insulating properties. A few metallic components such as injector nozzle, thermowells and refractory support rings, have to be used in the reaction vessel, but lifetimes have been limited. No material has been found that survives more than several months. The outer containment of the vessel is a metallic shell that is protected from excessively high temperature by the refractory lining on the inside and by forced flow of ambient air over the outer surface.
Heretofore, the need for suitable materials to contain high-temperature alkali-containing environments has not been fully met. What is needed is a suitable refractory material that solves the problem of degradation of containment materials for high-temperature alkali-containing environments such as black liquor and biomass gasifiers.